Om Namasivaya Nama:
Not-Self / Anaathma - Description:
Basically everything that constitute in this universe is Anaathma / Not-Self. Namely, The body, sense organs, Prana (Life energy), mind and ego and its modifications/changes etc,,Sense objects and organs and its pleasures etc.., the gross elements such as Pancha Boothas (ether, air, fire, water & earth), and the whole universe upto the Un-manifest, these constitute the Not-Self (Anaathma). In other words everything what we see and touch are Anathma
Everything is due to the effect of Maya (Avidya/Ignorance), from Mahat down to the gross body. Please note these as well the Maya itself not Self therefore these are un-real just like the mirage-water in the desert. We simply imagine due to illusion without the absolute understanding of real and truth. Therefore to know the Reality one must reject all un-real in other words Anaathman (Not-Self).
The Self (Athman) and its Nature:
Now Acharya says, i will tell you the Athman (The Supreme Self), knowing this. man will be freed from the bondage or life cycles (personality) created by his own un-intelligent ideas and attain liberation (Kaivalya Mukthi - Un-afflicted witness-er / motionless observer) for ever.
I, is the changeless common factor or consciousness as Individuality in us. This I is everywhere mainly the three states of consciousness such as waking, dream and deep sleep and also beyond the control of subtle 5 sheaths (Kosas - Annamaya/Food, Pranamaya/Vital-Air, Manomaya/Mind, Vignaanamaya/Intellectual and Anandamaya/Bliss). I is the one which is experiencing everything whichever stage the body is.Something that exists in the name of "Absolute Entity" The reason for the existence of eternal substratum is for the very awareness of the Ego, It is the witness of three states and it is distinct from the 5 Sheaths. However, these 3 states and 5 sheaths too are blocking or hindering us from realizing that Chaithanyam (Supreme Self).
Please click the below link for detailed discourse.